X-ray thickness gauge



Sept. 22 1953 w. N. LUNDAHL X-RAY THICKNESS GAUGE 3 Sheets-SheetZ Filed May 8. 1948 cent Screen Y ndem X-Pay Stabilizer oto-Maliz1blier INVENTOR Kit Indicator Walter 1v. Lundahl ATTOR Y Sept. 22, 1953 w. N. LUNDAHL 2,653,247

X-RAY THICKNESS GAUGE Filed May 8. 1948 5 Sheets-Sheet "3 G I L .95?

Selectz're Confral NTOR h alzer M Lundahl.

ATTORN Patented Sept.

UNI STATES PATENT" OFFICE 2,653,247 X RAY THICKNESS GAUGE wiltefN'. 'IiundahL'Pikesville, Md., assignor'to WstinghouseElectric Corporation, East Pittsliingh; Pa; a corporation of Pennsylvania Application May s, 1948,'Seri'al No. 25,953

My invention relatesto' X ray devicesland, in particular, relates tofarrangementsrer employing Xrays to measurelthe thickness of sheet materials.

In many cases where materials, as steel," for example, are being rolled or otherwise produced in the form of sheetsior'strips, it is necessary to gauge continuously the th'i'ckness or the material being turned out in order to insure uniformity of the product. While mechanical thickness gauges havebeen used, they have many undesirable properties and limitations; They almost invariably have. to make contact with the material, and thismay mark orm'ar it;' or they may be diificult to usebecause of 'i'ts'hig'h' temperature; or they may-strictly limitthe'i speed with whichthe material "may be" turnedbutof the producing apparatus. Since the absorption of X-nays increases with the'thickncssof a given material, attempts have been 'made to gauge thickness of s'heetmaterial being turned out by machines by irradiating them witha beam" of X-rays and measuring'the amountof 'X-ray absorption.

in providing an incident x-my'beamwmchis not subject to such variations intensityand wave length from timeto timefas woulds'eriously' in terfere with the" accuracybf the absorption' measurements. Unless the voltage employed to generate the' X-raysand the temperature v of the X-ray tube cathode are "maihtainedcons'tant' to an extremely highdegree of precision, seaside-n able apparent'variations in the absorption for a giventhickness of met'alare found. i

I have'found that theie'rrors' due to variations of the type just mentioned may be"e1imi'nated' bya system in which theinte'nsity of X-rays transmitted throughthesample is compared with the intensity of X-rays emanating fro'rn'the same source, or a substantially identical source, which are transmitted througha standard" body" or' sheet. In accordance "with the embodiment" of my inventiondes'cribedin detail herein, the above-mentionedmethodis carried out by irra diating the' sample-with anX-ray-beam emanat ing from one X-ray tube during one half cycle of an alternating-current ower supply, and irradiating a standard with X-rays emanating from an approximately identical X-ray tube energized from the same power supply during the other half cycle of the alternating voltage. This results in the production; during alternate half cycles of a powersu'p'ply, of two; different X'ray beams, one of whichfhas been transmitted through the sam'pleand'jth'e other of which has been transmittedthrough the'st'ahd'ard. The two transmitted beams then act on a suitable intensity measuring device to produce output currents in alternate half cycles of the power supply which are, respectively, measures of the intensity of the two transmitted beams.

By a suitable electronic'switching device, the sensitivity of the devicemeasuring the intensity of the transmitted beams is varied, by automatic volume control mthods'Qto produce a substantially constant 'outputburrent pulse from the measuring device during those half jcycles of the power supply in which the measuring device is irradiated by the more intense of the two X-ray beams. As a result, the output current of the same measuring device durir'igthose half cycles in which 'it measures the X-rays transmitted through the thicker of the two'mate'rials, is, for practical purposes, proportional to the ratio of the intensity of? the X-rays transmitted through the sample and the intensity of the X-rays transmitted through the standard. This ratio is substantially the same for a given sample and a given standard, over a wide range of variations in anode voltage-or filament voltage of the X-ray tubes, and is also substantially independent of any graduahvariations or drift in the sensitivity and. other characteristics of the measuring device. itselflf a system isthus obtained in which errors due to such variables as have been described in the preceding paragraphs, are eliminated. i I r I I Q Object of mrin sn io is a cor n o provide an arrangement for measuring the thickness of sheet and: other materials; by means of absorption of X-rays or other forms of radiation which is free irom errors which have interfered with the accuracy of prior. art measuring systems. I P V: v

Another object of 'my invention is, accordingly, to provide a thickness measuring arrangement which may produceaccurate measurementswithout"physi'ca11y contacting" themate'rial" being gauged. [I o Another object of'my invention'is to provid'e'a method of measuring the thickness of materials which is capable of accurate operation, even with materials which may be at extremely high temperatures.

Still another object of my invention is to provide an arrangement for measuring the absorption ability in a given material of X-ray and other forms of radiant energy in which the accuracy of the results is not vitiated by substantial variations in the electrical characteristics of the power systems supplying energy to the radiating source.

Still another object of my invention is to provide an arrangement for measuring the absorption ability for X-rays Or other radiation in test materials which is substantially unaffected by variations in the characteristics of the radiation-measuring device employed as a component thereof.

Still another object of my invention is to provide a circuit network for altering the wave form of a controlled quantity comprising alternate pulses of different amplitudes in such a way as to render a discriminator device of the type about to be referred to in the immediately following paragraph more sensitive to dinerences in the amplitude of said successive pulses.

Still another object of my invention is to provide a novel type of discriminator device which will produce across its output terminals a voltage which is proportional to the algebraic value of the difierence between successive pulses which are alternately of greater and smaller amplitude.

A still further object of my invention is to provide a novel type of relay and control circuit therefor for varying the position of a mechanism, such as a rolling mill, in response to the algebraic value of a controlled voltage such as the output voltage of the discriminator referred to in the immediately preceding paragraph.

Still another object of my invention is to provide a novel measuring device of high impedance for indicating the relative values of two electrical quantities.

Other objects of my invention will become apparent upon reading the following description, taken in connection with the drawings, in which:

Figure l is a block diagram showing the general layout of a thickness-measuring device for sheet metal passing through a rolling mill, which system embodies the principles of my invention referred to above;

Figs. 2 and 3 are schematic diagrams of the electrical circuits constituting the complete arrangement and showing in detail the separate units indicated in the block diagram of Fig. 1;

Figs. 4 through 9 are graphs illustrating wave forms produced at various points in the electronic circuits shown in detail in Figs. 2 and 3; and

Figs. 10 and 11 are diagrams illustrating the appearance of a fluorescent screen forming the indicating device of the indicator designated K in Figs. 1 and 2.

Certain features disclosed herein are shown and claimed in application Serial No. 758,014 of F. B. Marshall and J. W. Coltman filed about June 30, 1947 for an X-ray Thickness Gauge, and an application Serial No. 776,508 of R. L. Longini filed about September 27, 1947 for an X-ray Thickness Gauge.

Referring to Fig. l in detail, the general diagram of the system by which I have chosen to illustrate my invention shows a test sheet 3 emerging from the rolls 9'! and 98 of a rolling mill; the system embodying my invention being intended to indicate variations in the thickness of this test sheet while it emerges from the rolls and to automatically vary the setting of the rolls to maintain substantial constancy in said thickness. The components of the system above mentioned comprises a first X-ray source A emitting an X-ray beam which passes through the test sheet, and a second X-ray source '5 emitting a beam which passes through a master sheet or standard sheet H. The X-ray sources A and B are so positioned that their beams after passing through the sheets above mentioned irradiate a pick-up device (3 which produces an output current flowing to a combined amplifier and photomultiplier stabilizer, D. The output of the network D passes through a sensitivity control network 'E to a discriminator and indicator P which, in turn, impresses its output on a selective control network G. The output of the control network G is impressed on a reversing motor which controls the setting of the rolls Si and 58in the rolling mill. The output of the amplifier and photo-multiplier stabilizer D is likewise impressed on a novel type of indicator K. The above-mentioned system likewise embodies a stabilizer H for the two X-ray tubes A and B which acts to. maintain their anode currents always equal to each other.

Referring in detail to Fig. 2, a material of which the thickness is to be measured, which may, for example, be a band I of sheet steel passing through a rolling mill, is arranged to pass continuously in front of a shield 2 having an aperture through which an X-ray beam 3 i being projected from an X-ray tube A of suitable type. The X-ray tube A has its cathode excited from a transformer winding 5 and has voltage impressed on its anode from a second transformer winding 6 which may be energized by a primary winding 3 supplied from a suitable alternatingcurrent power source 8 not shown. A standard specimen or master sheet II, which may, for example, be a piece of sheet steel having the thickness which it is desired to have the sheet I duplicate, is arranged to be irradiated with X- rays through an aperture in a second shield l2 by an X-ray beam Hi from an X-ray tube B which is preferably, although not necessarily, a tube matched to the X-ray tube A. The tube B has its cathode supplied from a secondary winding l5 and its anode supplied from secondary windings it and m2. The windings 15, I6 and H32 are matched with the windings 5, 6 and Hill, respectively, and energized from a primary winding ll, which is preferably one matched with the transformer winding 7 and supplied from source 8.

The X-ray beams 3 and i3 are arranged to be coincident on a fluorescent screen 21, and a photo-sensitive tube 22 is arranged to be energized by the light emitted from the screen 2|. We have found a very practical arrangement in having the photo-sensitive tube 22 consist of a photo-multiplier tube of which many suitable types are well known today in the tube art. Such a tube produces an amplification of the photoelectric current from the cathode by the successive ejection of secondary electrons from a series of electrodes called dynodes. As is Well known, such tubes produce an output current of very substantial intensity, even when irradiated by relatively low intensity light. The amplification of such a device may bevaried by changing the voltage on one or more of the dynodes such as those indicated at 22-A1and 222B inFig. 2. The tube '22, as shown inithe drawings, .is purely symbolical, rather than a structural representation of such a photo-multipliertube.

The photo-sensitive cathode-of the tube 22 is preferably connected to the negative terminal of a voltage source 23 having its positive terminal grounded. The output .anode 22C of the tube 22 is. connected through asuitable resistor 24 to the lead 25 from the positive terminal of a second voltage source 2.6v which has its negative terminal grounded. This arrangement is such that the resistor 2c is traversed by a current which is, from instant to instant, .a replica of the intensity variations of the radiation incident upon the screen 2|.

Since the v'oltageapplied to the anodes of the X-ray tubes AandB is alternating, they irradiate the specimens 1 and M respectively, only in alternate one-half cycles of the alternating-voltage supply 8. The transformer windings I and. H are so poled that the :anode of the X-ray tube A is positive during one-half cycle and the anode of X-ray tube B is positive during the second half of each cycle. It will be seen to result from this that the beams 3 and i3, respectively, energize the photomultiplier tube 22 in alternate half cycles of the source 8. The output current from tube 22, flowing through resistor 24 therefore,

consists of a series of pulses, alternate members I of the series, respectively, being proportional in amplitude to the intensity of the radiations transmitted through the sample I and the standard H. These pulses are further amplified bya tube 33 resulting in an output measurable across a resistor 36.

Fig. l illustrates the variation with time of the current through the resistor 24,, the pulses M being measures of the intensity of the transmitted radiation in beam 3 passing through the sample l, and the pulses N being measures of the intensity of the radiation of the transmitted :beam passing through the standard ll on alternate half cycles of the power source .8. Fig. 5 shows the variation with time of the current through resistor 36.

The remainder, now about to be described, of the circuit network of Figs. 2. and 3 has the following purposes: First stabilizer Dimpresses. gain control voltages on the photo-sensitive tube .2201 such character that the larger of the two pulses of current M and N through resistor 36. are substantially always of the same: amplitude regardless of any variations. in absolute value of the transmitted radiation in beam I 3 and also regardless of any variations from timev to time in the characteristics of elements such as amplifier 33 or photo-sensitive tube 2.2;

Since one or the other of the amplitudes M and N is always equal to a constant quantity which amplitude is dependent only on thesetting of the. stabilizer, the magnitude of the smaller pulse is proportional to the ratio of transmission of the thicker of sheets I and -II to the transmission of the thinner of these two. The difierence between these two pulses is an exponential function of the difference in thickness of the sample and standard and is independent inmagnitude of the direction of this difierence. For small differences of thickness the difference in pulse magnitude is proportional to the. thickness difierence. The phase of the pulse difference indicates which pulse corresponds to the standard. and

which corresponds to the sample. Meter 21in.

6 Fig. 3 is so arranged that the phase of the difierence determines its direction of deflection.

Turning in detail to the circuit for effecting the above-mentioned results, a coupling capacitor 3! and a resistor 32 are connected between the anode 220 of the photo-sensitive tube 22 and ground, the common terminal of capacitor 31 and resistor 32 being coupled to a control electrode of the amplifier tube '53 having its cathode grounded through a cathode resistor 3'4 shunted by a capacitor 35. The anode of the tube 33 is connected through a resistor 36 to the positive lead 25. The foregoing connections are such as to produce in resistor 3'6 current pulses M and N which substantially duplicate, with reversed polarity, the output current pulses of the multiplier tube 22.

The anode of tube 33 is connected through a clipper network comprising adiodetfia connected in multiple with a channel comprising a resistor 362) connected to ground through a voltage divider see, the cathode of the tube 36a being connected to the tap-point on a variable bias-voltage source 350. The cathode of the tube 360, is likewise connected to ground through a capacitor 31 in series with a resistor 38 running. A variable tap-point on the resistor '38 is connected to the control electrode of an amplifier tube l! which. draws current from the positive lead '25 through a resistor did and which has its. cathode connected to ground through a resistor E2.

The output voltage of the photo-multiplier 22 being of the type shown in Fig. 4, its wave form will be altered by the tube 33 so that the output of the latter has the wave form illustrated in Fig. 5. The efiect of varyingthe setting of the:

simultaneously controlled variable taps on the tension dividers 3'5 and 38 is to clip off the lower portions of the voltage Waves in'Fig. 5 and produce a wave of the form illustrated in Fig. 6 on the input to amplifier M. The efiect of the capacitance and inductance inherent'in the network connected with amplifier ill is to produce at its output terminals a wave form of the type indicated in Fig. "7'. It will be noted that the ratio of the amplitude of the waves N in Fig. 6 is much larger than the ratio of the arm plitude or the corresponding waves M and N in Figs. 4 and 5, Thus the effect of the network in the sensitivity control device E which has just been described is to improve the sensitivity of the circuit by accentuating the percentage diiference between the input waves M and N and this sensitivity may be varied at willby altering the position of the variable tap points on the potential device 360 and 38;

Ihe output of the sensitivity control device E is impressed on the control electrodes of a pair of similar amplifier tubes it and tea tthrough a capacitor t5 and resistor 45a. The cathodes of the two tubes M and Ma are connected to each other through a resistor- 356' having a variable tap which is connected to ground. The anodes of the tubes M and lia are respectively connected to the positive line-wire .25 through equal resistors 5E and that. A second air of control electrodes in the tubes 44% and Ma are connected to opposite terminals of the secondary winding ll having its midpoint grounded and energized by a primary winding connected to the same voltage source it as that supplying current to theanodes of the X-ray tubes .A and B. The anodes of the tubes as and tea are connected to each other through an inductor Land adhcect-current meter I 21 having. a zero at thecenter of its scale.v

' stant.

. 7 V The voltage waves M and N impressed by the sensitivity control network E above described appear with this same sign on one set of control grids of the tubes 44 and 44a. However, the transformer secondary 41 impresses on the other set of control grids of those tubes voltages P and PA which are respectively 180 out of phase with each other, as indicated in Figs. 8 and 9. The tubes id and Mia are biased by the resistor 45b through which they are grounded so that they conduct current at only such times as the voltage waves P and N impressed on their control electrodes are both positive. Thus the tube Ma has the alternating voltage P and the pulsating voltage N both positive during one half cycle of the alternating voltage P as shown in Fig. 8 and the tube Ma has positive voltages PA and M impressed on its control electrodes simultaneously only during the other half cycles of the alternating-current secondary 47. Hence tube 34 will conduct current through the resistor 4K3 only when the X-ray soiuce B is energized by one half cycle of the alternating voltage source 8, and correspondingly the tube 64a will produce current flow in the resistor 46a only when the X-ray source A is energized by the alternating voltage source 8.

If the pulses M and N are equal to each other in amplitude at any time, the voltage drop produced in the resistor 46 will be equal to the voltage drop produced by current flow in the resistor 46a and the voltage difference between the anodes of the tube 44 and Ma will be an alternating one having a value which averages to zero over any in tegral number of cycles of the alternating voltage source 8. The needle on the meter 21 will then remain at the zero position since this alternating voltage can send no substantial current through the inductor L. However, if the pulses M (produced by X-ray source A irradiatin test sheet 1) are smaller in value than the pulses N, the voltage drop through the resistor ifia will be less than the voltage drop through the resistor 4t and there will be a net direct-current flowing through meter 27 from the anode of tube 44a to that of tube M; and this will be indicative of a test sheet i thinner than master sheet II. On the other hand, when the test sheet I is thicker than master sheet ll so that the amplitude of the pulse N is greater than that of the pulse M, there will likewise be a direct-current flow through meter 2? but it will be of opposite polarity and hence from the anode of the tube 44 to the anode of the tube 44a. Thus there will exist between the anodes of the tubes .4 and 4412 an output voltage which is proportional in magnitude and sign to the excess of the amplitude of pulses N over the amplitude of pulse M.

The network comprising the elements id through 4'! comprises the discriminator F generally indicated in Fig. 1, and the output voltage of this discriminator taken off between the anodes of the tubes 44 and Ma. has a value which varies in magnitude and sign with the algebraic difference between the amplitudes of the waves M and N in Fig. 7.

The portion of the stabilizer D not yet described has the property of insuring that, regardless of the absolute value of the transmitted radiation l3, and of changes in sensitivity of the photo-sensitive tube 22, the height of the larger of the pulses M and N which pass through the output resistor 36, remains substantially con- The tube 6| has its anode connected through a resistor 62 to the positive lead 25.

8 The anode oftube 6! is likewise connected to volume control electrodes 22A and 2213 in photosensitive tube22 through grounded resistors R1, R2, and to ground through a pi filter 63 comprising resistor R3 and capacitors C2 and C3. The cathode of the tube El is connected to ground through a cathode resistor 64 and its control electrode is connected through a capacitor 65 to the anode of amplifier tube 33. The control electrode of tube ti is likewise connected through a resistor 65 in multiple with a rectifier 6'! which may be of the diode vacuum tube type to the negative terminal or" a voltage source 68 having its positive terminal grounded.

ihe result of the above-described connections is that tube 6! is connected in shunt across filter 63 which derives its voltage from source 25 through resistor 62, and that filter d3 provides the operating voltage for the gain control electrodes 22A and 2213 in photo-multiplier tube 22. This operating voltage is adjusted by the magnitude of the pulses M and N.

The larger of pulses M and N in the output resistor 36 of amplifier tube 33 are impressed as a series of short positive pulses on the input electrode of tube 61, causing it to periodically discharge the capacitors of filter 63. Between these periodic discharges charge flows into the capacitors of filter 63 through resistor 62 from source 26. As a result, the voltage of capacitor C3 in filter 63 has a sawtooth ripple of small magnitude. Filter E3 operates thus until a decrease in thickness of sample I causes a pulse M or N, which is larger than its immediate predecessors to appeal on resistor 36. Such a pulse, let us say M, will impress a more positive voltage on resistor E6 and on the control electrode of tube 6|, causing the latter to become more conductive, and the capacitor C3 in filter 63 will discharge further. The potential impressed on the automatic volume control electrodes 22A and 22B of tube 22 thus becomes less positive and the current flow through resistor 2 decreases. Accordingly, the gain of the photo-multiplier tube 22 is reduced, and the current increase through resistor 36 which produced the original change is counteracted. Since tubes 33 and 5t, together with their attendant circuits, are designed to be extremely sensitive to small changes in voltage of their control electrodes, the net result is that the current flowing through resistor 3t during pulses M is reduced again substantially to the value it had before the above-mentioned corrective changes were set in action by current increase through resistor 35 during the M pulse phase.

It will be noted that the capacitor C3 of filter 63, connected between the gain control electrode 22B and ground, acquires its charge from the voltage source 26 through resistor 62, but is prevented from attaining the full source voltage by being periodically partially discharged by the current pulses through the control tube in accord ance with the magnitude of pulses M and N as described above. The resistor 62 is so proportioned relative to capacitor C3 that, while this charge on capacitor C3 gradually rises through resistor 62 during a half cycle of source 8, the amount of this increase in the voltage of capacitor C3 is small. Thus the gain of the photo-multiplier tube 22 is set by the control tube 65 to hold the pulse M or N at a constant level. If the pulse immediately following such an adjustment of the voltage of capacitor C3 is not materially greater than its predecessor it will not cause tube 6| to discharge that capacitor, and the gain of photomultipliertube is'left substantially at this'same value by that pulse. Thus the gain of the measuring tube 22 is controlled to maintain the greater of the twopulses M and N substantially constant. Though the signal current flowing through the resistor 38 consists of a series of pulses which fall substantially to zero near the end of every half cycle of voltage of source 8, the use of capacitor coupling between the photo-multiplier anode 22 and the amplifier tube 33 and likewise between the amplifier tube 33 and the control tube El results in the loss of a base reference level. Since a capacitor cannot pass direct current, the voltage output of the capacitor 55 would, in the absence of the rectifier tube 6?, consist of an alternation both positive and negative about a base line or zero level which would be determined by the areas and shapes of both pulses M and N. The rectifier s1, however, permits the passage to ground of current whenever its cathode is negative with respect to its anode, thus char ing the condenser 55 to a direct-current potential such that the cathode oi the rectifier 57 goes negative at the interval between pulses only by the very slight amount necessary to make up for the loss of charge through the resistor 66 during the positive swing oi the'pulse. Thus the voltage across the resistor 56 falls substantially to zero between pulses, and the voltages on the control grids of the tubes i! during the pulses are essentially measured individually from the true zero level.

Turning now to the operation of discriminator F to automatically regulate the rolling mill to maintain the thickness of test sheet I constant, the output-voltage of the discriminator F is connected to the input of a selective control network G also shown in Fig. 3. Its input terminals are spanned by a pair of equal capacitors H and 72 through a choke L! which are respectively in parallel with a pair of resistors i3 and Hi. The common terminal of theresistors '13 and M is connected to the negative terminal or the voltage soi'irce' -75 having its positive terminal connected through the energizing windings "El and m of a pair oi relays, which respectively close contacts 19; 8 bi-A and contacts 86!, 82 32A when said windings are energized by currents larger than a predetermined value. The windings "H and is are connected respectively to the anodes of a pair or amplifier tubes 83' and as having: theircathcdes connected through. variable taps to a voltage divider 85 which spans the source l5. The positive terminal of. the voltage source 15 is likewise connected through a resistor 75A. and. two similar networks each comprising a resistor 85 shunted by a. capacitor 81' to the respective anodes of a second pair of. amplifier tubes 8i; and 39. The eathodea of the tubes 88' and. S9 areconnected to a tap point on a pair" of. resistors 9 I", 92 which are: connected in series between the common terminal of the resistors 86 and the common terminal of the resistors 13' and. 'Hi. The anodes of the tubes at and 89 are respectivelyconnected through resistors 93 and 94 to controlelectrodes the tubes 83 and 84 and control electrodes in the tubes 83 and 59 are, respectively, connected through resistors 95 and 923 to the output termi'nals ofthe discriminator F.

The test sheet I is of the same thickness as master sheet H as long as' the voltage impressed by the output terminals of discriminator F is,

has already been pointed out, a pure alternating one; the control electrodes oi the tubes 88- and 3? will the same potential relative to their cathodes and the anode currents drawn by these bridging contacts.

tubes through the resistors 86 will be identical. Hence the voltages impressed in the control electrodes of the tubes 83 and 84 will likewise be identical and the anode currents of the latter tubes flowing respectively through the relays l? and 18 will be identical. The tap points on the potential divider 85 are initially adjusted so that the currents flowing through the energizing windings of the relays El and '18 are just insufficient to move their contactors it through 82 to closed position. It now the test sheet I becomes thicker than master sheet I i, anode of tube 44 has an average value which is positive relative to the anode of tube MA, as has been pointed out above. Hence this means that tube 89 W11 draw a reduced current through its resistor 86 and that the control electrode of tube 84 becomes more positive. Hence the voltage of the control electrodes of the tube 88 will be more positive than that in tube 39, and hence tube 8t will draw more current from the source 15 than will tube 89. Since tubes 88 and 85 both draw current from source F5 through resistors 15A, the current flowing from the source '15 through tube 34 and winding 13 will increase above the value above described as existing when the thickness of test sheet I is the same as that of master sheet ll When this increase reaches a preselected value corresponding to the tolerance allowed for sheet I, contacts of relay 18- will be moved to closed circuit-position. In short, as long as the radiation transmitted through the test sheet equals the radiation transmitted through the master sheet, relays ll and it will both remain in open circuit position. However, if the radiation transmitted through the test sheet exceeds by a predetermined tolerance that transmitted through the master sheet, one of the relays will close its bridgi'ng contacts while if the radiation transmitted through the: test sheet drops below that transmitted by the master sheet by a predetermined tolerance, the other of the relays will close its bridging contact.

Power from a direct-current source A iscontrolled by the bridging contacts 19 through 82A toconnect this source with one polarity to the field Winding of a direct-current motor 95A. and to energize the armature thereof, to rotate the latter' in one direction when the relay 18 closes the bridging contacts, and to rotate the latter in the opposite direction when the relay H closes its The motor 96A controls the spacing between the rolls 9?, 88 acting. on the sheet I and thereby determining the thickness of the latter. Thus as long as the thickness of the sheet I equals the thickness of the master sheet H, both the relays ll and 18 remain open circuited and the motor 86A remains deenergized. Howeverl ii the thickness of the roll sheet falls below that of the sample, the relay 18' will close its bridging contacts to rotate the motor 95A and increase the Spacing between the rolls 9'! and 98, whereas ifv the thickness of the rolled sheet is greater than that of the sample at any instant, the relay 1 1 will close its bridging contacts to rotate the motor 96A to decrease the spacingbetween. the rolls 9-! and 98.

Turning now to the X-ray tube stabilizer H, it is designed to maintain the anode current of the X-ray tubes A and. B constantly coupled to each other in spite of i any unwanted variations in their cathode-heating currents or the. like.

' The circuit of this X-ray tube stabilizer isthe joint invention of- R. L. Wright and W- N. Lunda-hl-andi separately described and claimed in 11- Patent No. 2,492,304, granted December 27, 1949 for X-Ray Tube Stabilizers.

In this stabilizer the cathodes of the X-ray tubes A and B are respectively connected to the grounded terminals of the transformer secondaries f and I6 which supply these tubes with anode voltage through windings IDI and I02 positioned on the magnetic cores of the respective transformer primaries I and H. In series with the windings NH and Hi2 are two equal resistors m3 and IM having their opposite terminals connected to the anodes of a pair of substantially identical diode rectifiers which may continuously be embodied in an electron tube IE5. The cathodes associated in the tube I05 respectively with the above-mentioned anodes are connected to ground through a pair of similar variable resistors fill and it which are respectively shunted by capacitors I69 and III).

The right-hand cathode of the double diode I95 is connected to the cathode of an amplifier tube iii having an anode supplied with current from a voltage source II2 through a winding IE3 positioned on a magnetic core IIBA to control the degree of saturation thereof. The control electrode of the tube III is connected to the left-hand cathode of the double diode W5 through a resistor Hit and a bias battery H5. The resistor i It. is shunted by an anti-hunting or feed-back circuit of the type described in the pending application of Charles T. Zavales, Serial No. 617,625, filed September 20, 1945, and assigned to the same assignee as the present invention. Inasmuch as such anti-hunting circuit per se forms no part of the present invention, it is believed unnecessary to describe such arrangement in detail herein, since reference thereto can be had from such copending application. Suffice it to say that such anti-hunting circuit, as indicated by the block diagram in Fig.

2, comprises a feed-back transformer having its primary winding connected in series with a winding IIE on the magnetic core II3A having the saturation controlled by the Winding H3 and with a primary winding II'I associated with the secondary winding 5 to supply heat and current to the cathode of th X-ray tube A, and also with a transformer H8 supplied from the alterhating-current source 8. The feed-back transformer is thus subjected to the voltage drop through the reactor windings I I3-I I6 and winding II! of the cathode heating transformer for X-ray tube A.

This voltage is then boosted by the feed-back transformer of the anti-hunt arrangement by a center tapped step-up secondary winding which is connected to a full-wave rectifier valve and to a resistor connected in parallel in the resistor lid through a filter arrangement including a capacitor. Under normal conditions in which no oscillations occur in the X-ray tube mil1iamperage and hence in the filament heating circuit for the cathode of X-ray tube A, no change occurs in the voltage impressed across resistor lid from the anti-hunt arrangement. If, however, oscillations should occur in the cathode circuit and in the milliamperage of th X-ray tube, a pulsating voltage appears across the resistor of the anti-hunt arrangement which pulsations are transmitted by the anti-hunt capacitor to resistor H4. This, in turn, causes this rapidly changing voltage across resistor II i to effect the control grid of tube I05 thereby reducing the magnitude of these oscillations since tube I05 affects the voltage across the resistors 12 I03 and IE4 included in the milliamperage energizing circuit for the X-ray tubes A and B.

It will be evident, upon examination, from the above description that the anode current of the Y-ray tube A will flow to earth through the resistor I03 and the channel comprising the right-hand diode in the tube I05 and the resistor Ifil which is in parallel with resistor I63. Similarly, the anode current of the X-ray tube B will flow to earth through the resistor Ids and the channel comprising the left-hand diode in the tube Hi5 and the resistor I08. It will be clear that when the anode current of the X-ray tubes A and B are equal to each other, the potential drops in the resistors IE3 and IM are equal and opposite to each other and correspondingly the potential difference between the opposite terminals of the resistors It! and I08 is zero. The latter potential difference, in series with the bias voltage source I i5 and the resistor I I4 is impressed between the cathode and the control electrode of the amplifier tube III and will fix the value of the direct current in the winding II3 which controls the saturation of the magnetic core for the winding H6. Th winding H6 accordingly constitutes a saturable reactor controlled in value by the direct current through the tube III and variation of the latter will accordingly control the voltage impressed on the primary winding II! from the source I it. The net result of the foregoing situation is that the potential difference between the opposite terminals of the resistors I67 and H98 will control th cathode temperature of the X- ray tube A. The initial value of the heating current for the cathode of either the tube A or B may be initially adjusted by varying the voltage sources II2 or H5 or by varying the tap point on the transformer II3 so that the anode currents of the X-ray tubes A and B have the desired equal values at Which they are to be maintained. If thereafter the anode current of the tube A in creases above the value of the anode current of the tube B, a potential difference will be produced across the outside terminals of the resistors I07! and IE8 which will make the control grid of the tube I II less positive, thereby decreasing current flow through the winding I I3. This winding is so poled as to decrease the saturation in the magnetic core for the winding I I6, thereby increasing its reactance and decreasing the heating current for the cathode of X-ray tube A. In consequence the anode current of the tube A will decrease to substantial equality with that of the X-ray tube B.

Should the anode current of the X-ray tube A become smaller than that of the X-ray tube B, the opposite effects on the amplifier tube I I I and the reactance of the winding MB to those above described will obviously follow, with the result that the anode current of the X-ray tube A Will be raised to equality with that of the X-ray tube B. The stabilizer H is thus an effective agency to maintain equality of currents at all times through two electron tube circuits such as those of the X-ray tubes A and B.

It will be noted that the interposition of the diode rectifiers in tube I ahead of the resistors 1 It? and IE8 insures that any alternating current due to distributed capacitance between the Windings of the transformers supplying power to X- ray tubes A and B will be excluded from eifect on the grid potential of the amplifier tube III. The capacitors I09 and I I0 filter out any ripples in the voltages impressed by the anode currents of the two X-ray tubes A and B across the r sistors I61 and I08,

It is desirable in order to give-a visual indication of any failure of thecontrol system to, main tain equality of thickness of the rolled sheet I'- with the master sheet H, and also to assist insetting the voltage of tubes-A and B, by adjusting autotransformer 8-, so that for the desiredthiclrnessof sheet ithe photo multiplier tube C will operate at its most sensitive range, to have a sim-ple'visualindication of the value of the voltage being applied to the electrode 223 in the photo multiplier 22. While a vacuum tube voltmeter might seen ideal from a technical stand point for this purpose, such an instrument is relatively expensive and difficult to maintain in adjustment. have accordingly provided the indicator K of a novel type for determination of the value of the above-'mentioned voltage. For this purpose, I have employed what may be termed a twin indicator electron ray tube of which the GAFG-G is an example new on the market.- To suit the present purposes, this measuring. device must have an input circuit of relatively high impedance. Thus I connect a pair of resistors i32,. i33 between the electrode 22A: and ground and connect their common terminal to one of the deflector electrodes of the tube l3 I The cathode of the tube 53'! is grounded and its anode is connected through a resistor E3 1 tdany'convenient source of positive potential such as the positiveli'ne 25. Between this positive line and the cathode; a resistor 35 having a variable tap is connected to the other deflecting electrode of the tube i31 An ordinary directcurrent voltmeter I 36' is connected to measure the voltage of the last-mentioned deflector electrod'e and the cathode.

The tube I31 operates by bombarding a fluoresc'ent screen incorporated. therein with electrons from itscathode, to produce two bright and two dark sectorson this screen. Vfhen the voltages impressed on its two deflector electrodes are equal to each other, the two dark sectors are of approximately equal area and have the appearance shown in Fig. On the other hand, when one of the voltages is greater than the other, the areas of the twodark sectors are diiierent so that they have an appearance such as is indicated in Fig. 11. In order to measure the voltage applied between the electrode 22A and ground, the variable tap onthe resistor I35 is moved until the screen of the tube i-tl has the appearance of Fig. 10 andthe value of the voltage is then read off from the voltmeter its.

It may be noted that the photo-multiplier stabilizer D protects the photo-multiplier tube C; should sheet I be removed, by reducing the potential of electrodes 22A and 225 so that injurious currents cannot flow.

While I have applied this voltage-measuring device in connection with the X-ray thickness gauge described in this application, it is a measuring instrument of general applicability to many purposes which will be evident to those skilled in the art. For instance, the relays El and EB may control selectivity equipment of types well known in the art for segregating sheets above or below a predetermined thickness into separate piles or the like, instead of for operating the roll screw-down motor 9%.

I claim as my invention:

1. In an I i-ray thickness gauge for measuring material thickness of the type having means for generating two independent X-ray beams, light producing means responsive to the intensity of the respective X-ray beams impinging thereon 7 1'4 after passage-through a material ofjknown thickmess and a-material of unknown thickness, light sensitive means responsive to said light produc- ,ing means and operable to produce a pulsating voltage corresponding to any difference in the intensity of said X-ray beams as a measure of vthe deviationin the thickness of the unknown material from that of' the material of known thickness, the combination with said light sensitive means of a discriminator arrangement for producing an outputvoltage responsive in magnitude and sign to the relative amplitudes of alternately succeeding pulses produced by said light sensitive means; said discrimination arrangement comprising. an inleading wire for connection to said light sensitive means and to the positive terminal of a direct-current source, a pair of electron tubes each embodying a set of principal electrodes and two control electrodes, means for connecting the principal electrodes of both said tubes in series with said inleading wire and direct-current source through a pair of resistors having a common terminal, means for connecting one of said electrodes in each said tube to the opposite terminals of a source of alternating current, means for connecting the other said control electrodestogether to be energ'ized by the pulsating wave from said light sensitive means, and means for connecting the non lcomr'nonterminals of said resistors to a utilizavtion circuit.

2. In an X-ray thickness gauge for measuring material thickness of the type having means for generating two independent X-ray beams, light producing means responsive to the intensity of the respective X-ray beams impinging thereon after passage through a material of known thickness and a material: of unknown thickness, light sensitive means responsive to said light producing means and operable to produce a pulsating volt age corresponding to'any' difierence in the intensity of said X-ray beams as a measure of the deviation in the thickness of the unknown material from that of the material of known thickness, the combination with said light sensitive means of a discriminator arrangement for producing an output voltage responsive in magnitude and sign to the relative amplitudes of alternately succeeding pulses in a periodic voltage wave produced by said light sensitive means said discriminator arrangement comprising an inleading Wire for connection to said light sensitive means and to the positive terminal of a directcurrent source, a pair of electron tubes each embodyingi a set of principal electrodes and two control electrodes, meansfor connecting the principal electrodes of both said tubes in series with said inleading wire and direct-current source through, a pair of resistors having a common terminal, means for connecting one of said control electrodes in each said tube to the opposite terminals of a source of alternating current having a periodicity equal to that of the pulsating wave from said light sensitive means, means for connecting the other said control electrodes together to be energized by the pulsating wave from said light sensitive means, and means for connecting the non-common terminals of said resistors to a utilization circuit.

3. In an X-ray thickness gauge for measuring material thickness of the type having means for generating two independent X-ray beams, light producing means responsive to the intensity of the respective X-ray beams impinging thereon after passage through a material of known thickness and a material of unknown thickness, light sensitive means responsive to said light producing means and operable to produce a pulsating voltage corresponding to any difference in the intensity of said X-ray beams as a measure of the deviation in the thickness of the unknown material from that of the material of known thickness, the combination with said light sensitive means of a discriminator arrangement for producing an output voltage responsive in magnitude and sign to the relative amplitudes of alternately succeeding pulses in a periodic direct-current voltage wave produced by said light sensitive means said discriminator arrangement comprising an inleading wire for connection to said light sensitive source, a pair of electron tubes each embodying a set of principal electrodes and two control electrodes, means for connecting the principal electrodes of both said tubes in series with said inleading wire and direct-current source through a pair of resistors having a common terminal, means for connecting one of said electrodes in each said tube to the opposite terminals of a source of alternating current having a periodicity equal to that of said pulsating source from said light sensitive means, means for connecting the other said control electrodes together to be energized by the pulsating wave from said light sensitive means, and means for connecting the noncommon terminals of said resistors to a utilization circuit.

4. In an X-ray thickness gauge for measuring material thickness of the type having means for generating two independent X-ray beams, light producing means responsive to the intensity of the respective X-ray beams impinging thereon after passage through a material of known thickness and a material of unknown thickness, light sensitive means responsive to said light producing means and operable to produce a pulsating voltage corresponding to any difference in the intensity of said Y-ray beams as a measure of the deviation in the thickness of the unknown material from that of the material of known thickness,

the combination with said light sensitive means of a discriminator arrangement for producing an output voltage responsive in magnitude and sign to the relative amplitudes of alternately succeeding pulses in a periodic voltage wave produced by said light sensitive means said discriminator arrangement comprising an inleading wire for connection to said light sensitive means and to the positive terminal of a direct-current source, a pair of similar electron tubes each embodying a set of principal electrodes and two control electrodes, means for connecting the principal electrodes of both said tubes in series with said inleading wire and direct-current source through a pair of resistors having a common terminal, means for connecting one of said control electrodes in each said tube to the opposite terminals of a source of alternating current having a periodicity equal to that of said pulsating source from said light sensitive means, means for connecting the other said control electrodes together to be energized by the pulsating wave from said light sensitive means, and means for connecting the non-common terminals of said resistors to a utilization circuit.

5. In an X-ray thickness gauge for measuring material thickness of the type having means for generating two independent X-ray beams, light producing means responsive to the intensity of the respective X-ray beams impinging thereon after passage through a material of known thickness and a material of unknown thickness, light sensitive means responsive to said light producing means and operable to produce a pulsating voltage corresponding to any difference in the intensity of said X-ray beams as a measure of the deviation in the thickness of the unknown material from that of the material of known thickness, the combination with said light sensitive means of a discriminator arrangement for producing an output voltage responsive in magnitude and sign to the relative amplitudes of alternately succeeding pulses in a periodic voltage wave produced by said light sensitive means said discriminator arrangement comprising an inleading wire for connection to said light sensitive means and to the positive terminal of a direct-current source, a pair of electron tubes each embodying a set of principal electrodes and two control electrodes, means for connecting the principal electrodes of both said tubes in series with said inleading wire and direct-current source through a pair of equal resistors having a common terminal, means for connecting one of said control electrodes in each said tube to the opposite terminals of a source of alternating current having a periodicity equal to that of said pulsating source from said light sensitive source, means for connecting the other said control electrodes together to be energized by the pulsating wave from said light sensitive means, and means for connecting the non-common terminals of said resistors to a utilization circuit.

6. In an X-ray thickness gauge for measuring material thickness of the type having means for generating two independent X-ray beams, light producing means responsive to the intensity of the respective X-ray beams impinging thereon after passage through a material of known thickness and a material of unknown thickness, light sensitive means responsive to said light producing means and operable to produce a pulsating voltage corresponding to any difference in the intensity of said X-ray beams as a measure of the deviation in the thickness of the unknown material from that of the material of known thickness, the combination with said light sensitive means of a discriminator arrangement for producing an output voltage responsive in magnitude and sign to the relative amplitudes of alternately succeeding pulses in a periodic directcurrent voltage wave produced by said light sensitive means said discriminator arrangement comprising an inleading wire for connection to said light sensitive means and to the positive terminal of a direct-current source, a pair of similar electron tubes each embodying a set of principal electrodes and two control electrodes, means for connecting the principal electrodes of both said tubes in series with said inleading wire and directcurrent source through a pair of equal resistors having a common terminal, means for connecting one of said control electrodes in each said tube to the opposite terminals of a source of alternating current having a periodicity equal to that of said pulsating source from said light sensitive means, means for connecting the other said control electrodes together to be energized by the pulsating wave from said light sensitive means, and means for connecting the non-common terminals of said resistors to a utilization circuit.

7. In an Y-ray thickness gauge for measuring material thickness of the type having means for generating two independent X-ray beams, light producing means responsive to the intensity of the respective X-ray beams impinging thereon after passage through a material of known thickness and a material of unknown thickness, light sensitive means responsive to said light producing means and operable to produce a pulsating voltage corresponding to any diiference in the intensity of said X-ray beams as a measure of the deviation in the thickness of the unknown material from that of the material of known thickness, the combination with said light sensitive means of a sensitivity control network for accentuating the relative differences between su-ccessive pulsations in a periodic voltage wave produced by said light sensitive means said sensitivity control network comprising an electron tube having a pair of principal electrodes and a control electrode, means for causing direct current flow between said principal electrodes from a source independent of the pulsating voltage wave from said light sensitive means, an input circuit for impressing said pulsations from said light sensitive means on said control electrode which embodies a variable resistor in series with a network comprising a rectifier which is shunted by a source of direct-current voltage separate from that supplied to said principal electrodes together with a resistor in series with said direct-current source, and means connecting the output circuit of said sensitivity control network to a utilization circuit.

8. In an X-ray thickness gauge for measuring material thickness of the type having means for generating two independent X-ray beams, light producing means responsive to the intensity of the respective X-ray beams impinging thereon after passage through a material of known thickness and a material of unknown thickness, light sensitive means responsive to said light producing means and operable to produce a pulsating voltage corresponding to any difference in the intensity of said X-ray beams as a measure of the deviation in the thickness of the unknown material from that of the material of known thickness, the combination 18 with said light sensitive means of a sensitivity control network for accentuating the relative diiferences between successive pulsations in a periodic voltage wave produced by said light sensitive means said sensitivity control network comprising an electron tube having a pair of principal electrodes and a control electrode, means for causing direct current flow between said principal electrodes from a source independent of the pulsating voltage wave from said light sensitive means, an input circuit for impressing said pulsations from said light sensitive means on said control electrode, said input circuit embodying a variable resistor in series with a capacitor and with a network comprising a rectifier which is shunted by a source of direct-current voltage separate from that supplied to said principal electrodes together with a resistor in series with said direct-current source, and means connected to the output circuit of said sensitivity control network to a utilization circuit.

WALTER N. LUNDAHL.

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